package org.apache.lucene.search.function;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import org.apache.lucene.search.Explanation;

/**
 * Expert: represents field values as different types.
 * Normally created via a 
 * {@link org.apache.lucene.search.function.ValueSource ValueSuorce} 
 * for a particular field and reader.
 *
 * @lucene.experimental
 * 
 *
 */
public abstract class DocValues {
    /*
     * DocValues is distinct from ValueSource because
     * there needs to be an object created at query evaluation time that
     * is not referenced by the query itself because:
     * - Query objects should be MT safe
     * - For caching, Query objects are often used as keys... you don't
     *   want the Query carrying around big objects
     */

    /**
     * Return doc value as a float. 
     * <P>Mandatory: every DocValues implementation must implement at least this method. 
     * @param doc document whose float value is requested. 
     */
    public abstract float floatVal(int doc);

    /**
     * Return doc value as an int. 
     * <P>Optional: DocValues implementation can (but don't have to) override this method. 
     * @param doc document whose int value is requested.
     */
    public int intVal(int doc) {
        return (int) floatVal(doc);
    }

    /**
     * Return doc value as a long. 
     * <P>Optional: DocValues implementation can (but don't have to) override this method. 
     * @param doc document whose long value is requested.
     */
    public long longVal(int doc) {
        return (long) floatVal(doc);
    }

    /**
     * Return doc value as a double. 
     * <P>Optional: DocValues implementation can (but don't have to) override this method. 
     * @param doc document whose double value is requested.
     */
    public double doubleVal(int doc) {
        return floatVal(doc);
    }

    /**
     * Return doc value as a string. 
     * <P>Optional: DocValues implementation can (but don't have to) override this method. 
     * @param doc document whose string value is requested.
     */
    public String strVal(int doc) {
        return Float.toString(floatVal(doc));
    }

    /**
     * Return a string representation of a doc value, as required for Explanations.
     */
    public abstract String toString(int doc);

    /**
     * Explain the scoring value for the input doc.
     */
    public Explanation explain(int doc) {
        return new Explanation(floatVal(doc), toString(doc));
    }

    /**
     * Expert: for test purposes only, return the inner array of values, or null if not applicable.
     * <p>
     * Allows tests to verify that loaded values are:
     * <ol>
     *   <li>indeed cached/reused.</li>
     *   <li>stored in the expected size/type (byte/short/int/float).</li>
     * </ol>
     * Note: implementations of DocValues must override this method for 
     * these test elements to be tested, Otherwise the test would not fail, just 
     * print a warning.
     */
    Object getInnerArray() {
        throw new UnsupportedOperationException("this optional method is for test purposes only");
    }

    // --- some simple statistics on values
    private float minVal = Float.NaN;
    private float maxVal = Float.NaN;
    private float avgVal = Float.NaN;
    private boolean computed = false;

    // compute optional values
    private void compute() {
        if (computed) {
            return;
        }
        float sum = 0;
        int n = 0;
        while (true) {
            float val;
            try {
                val = floatVal(n);
            } catch (ArrayIndexOutOfBoundsException e) {
                break;
            }
            sum += val;
            minVal = Float.isNaN(minVal) ? val : Math.min(minVal, val);
            maxVal = Float.isNaN(maxVal) ? val : Math.max(maxVal, val);
            ++n;
        }

        avgVal = n == 0 ? Float.NaN : sum / n;
        computed = true;
    }

    /**
     * Returns the minimum of all values or <code>Float.NaN</code> if this
     * DocValues instance does not contain any value.
     * <p>
     * This operation is optional
     * </p>
     * 
     * @return the minimum of all values or <code>Float.NaN</code> if this
     *         DocValues instance does not contain any value.
     */
    public float getMinValue() {
        compute();
        return minVal;
    }

    /**
     * Returns the maximum of all values or <code>Float.NaN</code> if this
     * DocValues instance does not contain any value.
     * <p>
     * This operation is optional
     * </p>
     * 
     * @return the maximum of all values or <code>Float.NaN</code> if this
     *         DocValues instance does not contain any value.
     */
    public float getMaxValue() {
        compute();
        return maxVal;
    }

    /**
     * Returns the average of all values or <code>Float.NaN</code> if this
     * DocValues instance does not contain any value. *
     * <p>
     * This operation is optional
     * </p>
     * 
     * @return the average of all values or <code>Float.NaN</code> if this
     *         DocValues instance does not contain any value
     */
    public float getAverageValue() {
        compute();
        return avgVal;
    }

}
